Thick-film resistors are employed in electronic circuits to provide small, inexpensive resistors of a wide range of values. Such resistors are typically formed by printing, such as screen printing, a thick-film resistive paste or ink on a substrate, which may be a printed wiring board (PWB), flexible circuit, or a ceramic or silicon substrate. Thick-film inks used on organic printed wiring board constructions are typically composed of an electrically conductive material, various additives used to favorably affect the final electrical properties of the resistor, an organic binder and an organic vehicle. After printing, the thick-film ink is typically heated to dry the ink and convert it into a suitable film that adheres to the substrate. When a polymer thick-film (PTF) ink is used, the organic binder is a polymer matrix material and the heating step serves to remove the organic vehicle and cure the polymer matrix material.
The electrical resistance of a thick-film resistor is dependent on the precision with which the resistor is produced, the stability of the resistor material, and the stability of the resistor terminations. The “x” and “z” dimensions (the width and thickness, respectively) of a rectangular PTF resistor are largely determined by a screen printing process, while the “y” dimension (the electrical length of the resistor), which is established by a separation distance of two terminations, can be designed to achieve a desired resistance. Conventional screen printing techniques generally employ a template with apertures bearing the image of the resistor to be created. The template, referred to as a screening mask, is placed above and in close proximity to the surface of the substrate on which the resistor is to be formed. The mask is then loaded with a PTF resistive ink, and a squeegee blade is drawn across the surface of the mask to press the ink through the apertures and onto the surface of the substrate. Copper or other metal terminations are typically formed prior to deposition of the ink by additive plating or subtractive etching of a copper foil or a copper foil and electrolytically plated copper. Both of these processes are capable of achieving a high level of edge definition that enables accurate determination of the electrical length (y) of the resistor.
Compared to many other deposition processes, screen printing is a relatively crude process. As a result, screen-printed PTF resistors are typically limited to dimensions of larger than about one-tenth millimeter. While the y dimension (electrical length) of a screen-printed PTF resistor can be accurately determined by using appropriate processes to form the terminations, control of the x and z (width and thickness) dimensions of a PTF resistor is fundamentally limited by the relatively coarse mesh of the screen and by ink flow after deposition. Control of resistor dimensions is further complicated by the variability of the surface on which the resistive ink is printed, due in large part to patterned metal interconnects for these resistors having typical thicknesses of about ten to thirty-five micrometers—that is, the print surface is non-planar. The non-planar board surface affects a uniform squeegee action across the surface, resulting in imperfect printing of the screen image and non-uniform deposition of the resistor ink. In addition, non-uniform temperatures across the body of the resistor during curing may also introduce variations in the resistivity. Consequently, the distributions of resistor values when fabrication of a circuit board is completed generally have unacceptable mean values and coefficients of variation, requiring empirical adjustments of the resistor dimensions during the process design stage, particularly for complex circuits with a wide range and number of different resistance values. Such empirical adjustments add time and cost to the resistor printing process, as new artwork must be generated, new screens fabricated, and additional prototypes fabricated to validate the changes.
From the above, it can be seen that what is needed is a method for forming PTF resistors with more predictable and consistent resistance values, without the need for empirical adjustments to align the actual mean values with the desired target resistance values.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.